Current Protein and Peptide Science - Volume 14, Issue 1, 2013

Microglia serve in the surveillance, maintenance and protection of the central nervous system (CNS) homeostasis and functionality. The process of transformation from their house-keeping status to reactive phenotypes upon CNS challenges is known as microglial activation. It comes also with dramatic changes in protein expression and release. Activated microglia may thereby mount a rather homogenous response, with all cells of an affected local population simultaneously upregulating the same cell surface receptors or synthesizing an identical set of soluble messengers. Yet there is increasing evidence for a constitutive heterogeneity of microglia by and within CNS regions—largely being based on protein expression as well as activities and pointing to distinct functional capacities as to microglial subtypes. Inductions of proteins with key functions in antigen presentation and inflammation, like major histocompatibility complex (MHC) class I or II molecules and tumor necrosis factor (TNF) α, reveal that among a pool of activated microglia individual cells can differ by actual contributions. While MHC I induction can be appropriately triggered as a panpopulational response, only a subset would organize for TNFα production. Similarly, MHC II expression seems to be confined to a microglial subpopulation, and disposal of myelin either under normal conditions or its removal upon CNS damage appear to be duties of specialized cells, partially with complementary distribution. Discrete synthesis of immunoregulatory proteins would thus assign a master control to certain microglia, while tasks in the clearance of endogenous material and in professional antigen presentation could be sequestered to avoid collision of incompatible functions.

Microglia are monocyte-macrophage lineage cells, while other glial cells are neuroectodermal origin. Accumulation of microglia is commonly observed around degenerating neurons. There, microglia produce a variety of factors and function both neurotoxic and neuroprotective. Thus, accumulation of glia in various neurological disorders is not a static scar, gliosis, but more actively involved in degeneration and regeneration as neuroinflammation. We have shown previously that the most neurotoxic factor from activated microglia is glutamate, and that the suppression of glutamate release from microglia results in amelioration of disease progression in animal models of neurodegenerative disorders. On the other hands, when exposed to harmful stimuli, neurons also produce various factors as “help me” signals. Recently, we found that a CX3C chemokine, fractalkine (FKN), and interleukin-34 (IL-34) were secreted from damaged neurons. FKN and IL-34 differently activated microglia to rescue neurons by upregulating phagocytosis of toxicants or damaged debris, and production of anti-oxidant enzyme. The bi-directional interaction between neurons and microglia is important for understanding of chronic neuroinflammation, and gives us clues for future therapeutic strategy against neurodegenerative disorders.

Reciprocal interactions between cells caused by release of soluble factors are essential for brain function. So far, little attention has been paid to interactions between neurons and glia. However, in the last few decades, studies regarding such interactions have given us some important clues about possible mechanisms underlying degenerative processes in neurological diseases such as Alzheimer's disease and Parkinson's disease. Activated microglia and markers of inflammatory reactions have been consistently found in the post-mortem brains of diseased patients. But it has not been clearly understood how microglia respond to neurotransmitters released from neurons during disease progression. The main purpose of this review is to summarize studies performed on neurotransmitter receptor expression in microglia, and the effects of their activation on microglial-mediated neuroinflammation. A possible mechanism underlying transmitter- mediated modulation of microglial response is also suggested. Microglia express receptors for neurotransmitters such as ATP, adenosine, glutamate, GABA, acetylcholine, dopamine and adrenaline. Activation of GABA, cholinergic and adrenergic receptors suppresses microglial responses, whereas activation of ATP or adenosine receptors activates them. This latter effect may be due primarily to activation of a Ca2+-signaling pathway which, in turn, results in activation of MAP kinases and NFkB proteins with the release of proinflammatory factors. However, glutamate and dopamine are both pro- and anti-inflammatory depending on the receptor subtypes expressed in microglia. More detailed studies on downstream receptor-signaling cascades are needed to understand the roles of neurotransmitters in controlling neuron-microglia interactions during inflammatory processes in disease progression. Such knowledge may suggest new methods of treatment.

Toll-like receptors (TLRs) are type I transmembrane signaling molecules that are expressed in cells of the innate immune system. In these cells, TLRs function as pattern recognition receptors (PRR) that recognize specific molecular patterns derived from microorganisms. Upon activation, TLRs trigger a cascade of intracellular signaling pathways in innate immune cells, leading to the induction of inflammatory and innate immune responses, which in turn regulate adaptive immune responses. In the nervous system, different members of the TLR family are expressed on glial cells (astrocytes, microglia, oligodendrocytes, and Schwann cells) and neurons. Recently, increasing evidence has supported the idea that TLRs also recognize endogenous molecules that are released from damaged tissue, thereby regulating inflammatory responses and subsequent tissue repair. These findings imply that TLRs on glial cells may also be involved in the inflammatory response to tissue damage in the nervous system. In this review, we discuss recent studies on TLR expression in the cells of the nervous system and their roles in acute neurological disorders involving tissue damage such as strokes, traumatic spinal cord and brain injuries, and peripheral nerve injuries.

The plasma membrane Na+/Ca2+ exchanger (NCX) is a bidirectional ion transporter that couples the translocation of Na+ in one direction with that of Ca2+ in the opposite direction. This system contributes to the regulation of intracellular Ca2+ concentration via the forward mode (Ca2+ efflux) or the reverse mode (Ca2+ influx). We have previously demonstrated that the Ca2+ paradox, an in vitro reperfusion model, causes the sustained activation of the reverse mode of the NCX, the disruption of Ca2+ homeostasis, and subsequent delayed apoptotic-like death in astrocytes. In addition, we found that the nitric oxide (NO)-cyclic GMP signaling pathway inhibits Ca2+ paradox-mediated astrocyte apoptosis, while a high concentration of NO induces cytotoxicity. In this way, Ca2+ and NO may work together in the pathogenesis of several cells in the central nervous system. Concerning the role of NCX in NO cytotoxicity, we have found, using the specific inhibitor of NCX 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), that NCX is involved in NOinduced cytotoxicity in cultured microglia, astrocytes, and neuronal cells. This review summarizes the pathological roles of the NCX as a new target for NO-mediated cellular toxicity, based on our studies on NO-NCX-mediated glial toxicity.

Therapeutic hypothermia is a useful intervention against brain injury in experimental models and patients, but its therapeutic applications are limited due to its ill-defined mode of action. Glia cells maintain homeostasis and protect the central nervous system from environmental change, but after brain injury, glia are activated and induce glial scar formation and secondary injury. On the other hand, therapeutic hypothermia has been shown to modulate glial hyperactivation under various brain injury conditions. We considered that knowledge of the effect of hypothermia on the molecular profiles of glia and on their phenotypes would improve our understanding of the neuroprotective mechanism of hypothermia. Here, we review the findings of recent studies that examined the effect of hypothermia on proteome changes in reactive glial cells in vitro and in vivo. The therapeutic effects of hypothermia are associated with the inhibition of reactive oxygen species generation, the maintenance of ion homeostasis, and the protection of neurovascular units in cultured glial cells. In an animal model, a distinct pattern of protein alterations was detected in glia following hypothermia under ischemic/reperfusion conditions. In particular, hypothermia was found to exert a neuroprotective effect against ischemic brain injury by regulating specific glial signaling pathways, such as, glutamate signaling, cell death, and stress response, and by influencing neural dysfunction, neurogenesis, neural plasticity, cell differentiation, and neurotrophic activity. Furthermore, the proteins that were differentially expressed belonged to various pathways and could mediate diverse phenotypic changes of glia in vitro or in vivo. Therefore, hypothermia-modulated glial proteins and subsequent phenotypic changes may form the basis of the therapeutic effects of hypothermia.

Glycosaminoglycans (GAGs) have crucial roles in cell-cell interaction and communication. The communication between sperm and egg during fertilization is the finest example of intercellular communication involving a proteincarbohydrate recognition system. GAGs, especially heparin, are implicated in various processes, such as capacitation, acrosome reaction (AR), and sperm nuclei decondensation by interacting with a wide range of proteins, leading to fertilization. Seminal plasma (SP) comprises of multiple proteins that bind to heparin and related GAGs. Heparin binding proteins (HBPs) originating from secretions of the male accessory sex glands are known to play a vital role during fertilization events. They interact with GAGs present in the female genital tract and enhance the subsequent zona pellucida-induced AR, and thus have been correlated with fertility in some species. Several carbohydrate and zona pellucida-binding proteins, many of which belong to the spermadhesin family, are identified as HBPs. Many studies have been documented about the potential physiological role of some HBPs in various steps of fertilization. However, there is insufficient knowledge about functions executed by various HBPs and their exact mechanism and pathways. An in-depth knowledge of HBPs and their role in fertilization is of fundamental importance to resolve biological pathways and protein interactions at the molecular level. This review surveys some of the relevant findings supporting the potential role of heparin and HBPs in reproduction. It also describes consensus heparin binding sites emerging from known literature on HBPs that interact with heparin.

Tissue engineering requires an ideal scaffold that will aid in the regeneration of the damaged tissues both structurally and functionally. Conventionally, polymeric nanofibrous scaffolds have been extensively used due to their structural similarity to the native extracellular matrix. Thus far, top-down approaches like electrospinning and phase separation have been predominantly used for the nanofiber fabrication. Recently, self-assembling peptide nanofibers (SAPNF) have been identified as promising scaffolds for tissue engineering applications. Molecular self-assembly of peptides, which is a bottom-up approach has laid foundations for the development of such novel scaffolds. Designer self-assembling peptides provide functional support as well as bio-recognition due to the presence of bioactive motifs embedded in them. However, there are certain limitations to both electrospun and SAPNF scaffolds in terms of synthesis, cues presented to the biological system and applications. Design of composite, hybrid scaffolds by super-positioning possible cues may result in effective functional tissue regeneration at multiple levels.